1. Field of the Invention
The present invention relates to a vertical incidence type vertical optical modulator grown on a substrate, device driving method and fabrication process thereof.
2. Description of the Related Art
The following is an example of prior art semiconductor laser structures (Kasahara et. al., Japanese Patent Kokai Publication No. 5-152674 (see FIG. 12)). In FIG. 12, reference numeral 101 designates a semi-insulative GaAs substrate; 102 designates an Si-doped mirror having alternating laminated layers of n-GaAs and n-AlAs; 103 designates an undoped Al0.25Ga0.75As layer; 104 designates an undoped InGaAs active layer; 105 designates an undoped Al0.25Ga0.75As layer; 106 designates a Be-doped mirror having laminated layers of p-GaAs and p-AlAs; 107 designates a p-GaAs (3 λ thick) layer; 108 designates a optical absorption layer composed of three InGaAs strain quantum well layers and a GaAs barrier layer (46 nm thick); and 109 designates an undoped GaAs layer (λ thick). Reference numeral 110 designates an Si-doped mirror in which n-GaAs layers (λ/4 thick, n=2×1018 cm−3) and n-AlAs layers (λ/4 thick, n=2×1018 cm−3) are alternately laminated in 24.5 cycles. Reference numeral 111 designates a SiN film, 114 and 112 designate an n-side electrode and 113 designates a p-side electrode.
The device is operated under the condition where the n-side electrode 114 is grounded and a positive voltage is applied to the p-side electrode 113 whereas a negative voltage being applied to the n-side electrode 112. Since an electric field is applied to the optical absorption layer 108 which is reversely biased, the band end is shifted to longer wavelengths owing to the Franz Keldysh effect and light from the InGaAs active layer 104 can be turned ON and OFF. By virtue of the arrangement in which the optical absorption layer 108 is disposed between the Be doped mirror 106 and the Si-doped mirror 110, nearly 100% absorption can be accomplished at the time of voltage application even though the actual film thickness of the optical absorption layer 108 is only 30 nm.
In the prior art, since the absorption coefficient of the optical absorption layer 108 is only 1,000 to 10,000 cm−1, the extinction ratio is increased by letting light reflect and reciprocate between the Be-doped mirror 106 and the Si-doped mirror 110. This approach, however, has presented the problem that the intensity of the out-going light is significantly low because the light modulated by the light absorption layer 108 reflects at the Si-doped mirror 110.
Other known vertical optical modulators are disclosed in Japanese Patent Kokai Publication No. 7-307464 (particularly in the paragraph No. 0010) and Japanese Patent Application No. 2000-275692.